Various types of batteries now available contain a shortproof separator between negative and positive poles. In recent years, lithium batteries have been attracting attention for applicability to cordless electronic equipment because of their high energy density, high electromotive force, and small self-discharge.
Materials of the negative pole of conventional lithium batteries include metallic lithium, lithium alloys with other metals, e.g., aluminum, organic materials capable of adsorbing a lithium ion, e.g., carbon or graphite, organic materials capable of occluding a lithium ion by intercalation and lithium ion-doped conductive polymers. Known materials of the positive pole of the lithium batteries include fluorinated graphite represented by the formula (CF.sub.x).sub.n, metal oxides (e.g., MnO.sub.2, V.sub.2 O.sub.5, CuO and Ag.sub.2 CrO.sub.4) and sulfides (e.g., TiS.sub.2 and CuS).
Lithium as a negative pole constituent of lithium batteries has high reactivity, and the electrolytic solution used in lithium batteries is a solution comprising, as an electrolyte, LiPF.sub.6, LiCF.sub.3 SO.sub.3, LiClO.sub.4 or LiBF.sub.4, and an organic solvent, e.g., ethylene carbonate, propylene carbonate, acetonitrile, .gamma.-butyrolactone, 1,2-dimethoxyethane or tetrahydrofuran. Therefore, should an abnormal current pass the battery due to external shortcircuiting or miss-connection, the battery temperature considerably rises, causing thermal damage to equipment in which the battery is integrated.
To avoid this, it has been proposed to use a separator comprising a porous film. For example, a porous film comprising polyethylene (hereinafter abbreviated as "PE") or polypropylene (hereinafter abbreviated as "PP") (see JP-A-60-23954 and JP-A-2-75151, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a porous film comprising a mixture of PE having an ordinary molecular weight and PE having a high molecular weight (see JP-A-2-21559), and a laminated porous film composed of different materials (see JP-A-62-10857 and JP-A-63-308866) have been proposed.
The purpose of using a single or laminated porous film as a battery separator resides in that the porous film interposed between a positive pole and a negative pole functions in a normal state to prevent shortcircuiting therebetween while suppressing electric resistance therebetween on account of its porous structure to maintain the battery voltage. On the other hand, where the inner temperature rises due to an abnormal current, the porous film changes into a non-porous film at a certain elevated temperature and thereby increases its electric resistance to shut the battery reaction whereby a further temperature rise can be prevented to ensure the safety.
A function of increasing electric resistance in case of a temperature rise due to an abnormal current to shut off a battery reaction thereby preventing a further temperature rise for security is generally called a shut-down (hereinafter abbreviated as "SD") function. An SD function is essential to separators for lithium batteries, etc.
In what follows, a temperature at which the resistivity of a battery separator reaches 200 .OMEGA..cm.sup.2 is referred to as "SD initiation temperature". If an SD initiation temperature is too low, an increase in resistance initiates on a slight temperature rise, making the battery impractical. If it is too high, security is insufficient. It is accepted for the time being that an SD initiation temperature is preferably from about 110.degree. to 160.degree. C., and more preferably from about 120 .degree. to 150.degree. C.
A battery separator is also demanded to retain the thus increased resistance up to an adequate temperature for security. The upper temperature limit up to which the increased resistance is retained will hereinafter be referred to as "heat resistance temperature", and the temperature latitude from an SD initiation temperature up to a heat resistance temperature, namely a difference between a heat resistance temperature and an SD initiation temperature, will hereinafter be referred to as "heat resistance range".
The heat resistance temperature of a separator may be regarded as an indication of a function of film form retention. If a separator is melted by heat, it no more retains its film shape and is broken, decreases its resistance, and loses its SD function. As a result, the positive and negative poles are short-circuited, resulting in a sudden temperature rise to cause thermal damage to the equipment. Therefore, from the safety consideration, it is required for a battery separator to have an SD initiation temperature ranging from about 110.degree. to 160.degree. C. and a high heat resistance temperature with a broad heat resistance range.
Besides the above-described SD function, it is basically required for a battery separator to have a low resistivity, high mechanical strength such as a tensile modulus, small unevenness in film thickness, and small variations in characteristics, such as resistivity.
Conventional porous films which can be used as a separator of lithium batteries include a porous PP film obtained by extruding PP at a high draft ratio (a quotient of a take-off speed of a film divided by an extrusion speed of a resin from a die), subjecting the extruded film to a heat treatment (annealing), followed by stretching (see JP-B-46-40119 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-B-55-32531, and U.S. Pat. Nos. 3,679,538 and 3,801,404); a porous PE film obtained by molding a composition comprising PE having a specific molecular weight and a specific molecular weight distribution, an inorganic fine powder, and an organic liquid into a film, extracting the inorganic fine powder and the organic liquid from the film, followed by stretching (see JP-B-59-37292); and a biaxially stretched porous film comprising PP and PE (see JP-A-50-111174).
According to the inventors' experiments, a battery separator comprising the above-described porous PP film has an SD initiation temperature as high as 170.degree. C. or more while that comprising the above-described porous PE film has a moderate SD initiation temperature of about 135.degree. C. but a heat resistance temperature of at most 145.degree. C. That is, either film has a need of further improvement for safety.
In addition, a porous PE film has a low tensile modulus of about 3200 kg/cm.sup.2 or less and is therefore liable to elongation when integrated into a battery, thus failing to increase the production rate. Further, a porous film comprising a mixture of PE having an ordinary molecular weight and high-molecular PE shows improvements in characteristics but has a heat resistance of about 150.degree. C. and a tensile modulus of about 3400 kg/cm.sup.2, which are still insufficient.
A separator composed of laminated porous films of different materials seems to have improved SD characteristics. However, since the positions of fine pores of each film do not correspond with each other when laminated, the fine pores fail to connect the surface side to the back side and has increased resistance as a result. Where the films are laminated with an adhesive, part of the numerous pores are clogged with the adhesive, which also results in an increase of resistance. Moreover, a laminated film has a so increased thickness, which goes against the contemporary trend to size reduction and increase of energy density of batteries.
According to JP-A-50-111174 supra, the biaxially stretched porous film comprising PP and PE disclosed therein becomes transparent on about 1-minute's immersion in methanol at room temperature. From the fact that a porous film used as a battery separator usually becomes transparent almost in the instant of being immersed in methanol, it appears that the porous film of JP-A-50-111174 has a low degree of porosity and accordingly too high electric resistance for practical use.
The inventors have continued extensive studies to overcome the above-described problems associated with the conventional techniques, i.e., the low heat resistance and insufficient mechanical strength of a separator comprising a porous PE film and the high SD initiation temperature of a separator comprising a porous PP film.
In the course of their study, they previously proposed a porous film separator comprising a mixture of PE and PP as disclosed in JP-A-4-206257. The proposed battery separator exhibits improved SD characteristics as compared with the conventional ones by virtue of the PE/PP combined use and application of a novel process for film production. Nevertheless, the PE/PP mixed porous film still has room for further improvements in, for example, reducing electric resistance and wrinkle.